Microstructure Of Micro-arc Oxidation Coatings Research Articles

Mechanistic analysis of improving the corrosion performance of MAO coatings on Ti-6Al-4V alloys by annealing

Titanium and its alloys find extensive application within the realms of dentistry and orthopedics due to their remarkable chemical stability. Nevertheless, they are still susceptible to corrosion in harsh environments. Micro-arc oxidation (MAO) emerges as a potent methodology for enhancing the corrosion resistance of titanium and its alloys, achieved through the generation of a ceramic stratum atop the material's surface. The corrosion resistance of MAO coatings in clinical applications is closely related to their microstructure. This manuscript systematically explores the effect of annealing temperature on the phase composition and microstructure of MAO coatings. Furthermore, the evolution of the surface properties and the crystal structure of coatings is meticulously discussed and illustrated. The results show that the corrosion resistance of coatings exhibits an ascending trajectory followed by a subsequent decline, coinciding with the augmentation of the annealing temperature. This behavior can be attributed to the structural evolution of coatings caused by the gradual increase of the annealing temperature. At the optimum temperature (400 °C), the annealing treatment leads to the crystallization of the coating as well as to a decrease in the surface porosity, resulting in the formation of a compact and continuous multilayer microstructure. The decrease in porosity can be attributed to the diffusion-reaction of oxygen from the air with titanium from the discharge channel (titanium oxide), resulting in the formation of TiO2 particles that effectively fill the pores. The decrease in corrosion resistance of the coatings after annealing at 800 °C is due to the phase transition of TiO2 which leads to the expansion of cracks inherent in the oxide layer to form A-H cracks. Therefore, this study shows that the microstructure of MAO coatings can be adjusted by post annealing treatment (400 °C) to obtain a desirable oxide layer with appropriate porosity, crystallinity and chemical composition, thus improving its corrosion resistance.

Influence of electrolyte composition on microstructure and properties of coatings formed on pure Ti substrate by micro arc oxidation

The aim of this work is to study how electrolyte composition incorporates into the prepared micro arc oxidation (MAO) coatings on titanium in electrolytes containing K2ZrF6 or K2TiF6 and modify the surface characteristics. The microstructure of MAO coatings were characterized by scanning electron microscope (SEM), energy dispersive X-ray spectrometry (EDS) and X-Ray Diffractometer (XRD). Vickers hardness, corrosion resistance and wear resistance of these coatings were evaluated. The obtained results showed that arcing phenomenon could be affected by the electrolyte composition, and it occurred with short arc starting time and low arc starting voltage in the solution with K2TiF6 addition, resulting in an increasing growth rate of the coating. Compared with the MAO coating prepared in the base solution, the MAO coating formed in electrolyte with K2ZrF6 or K2TiF6 additives showed a self-sealing microstructure, exhibiting improved hardness. The ZrO2-modified MAO coating showed an excellent tribological behavior and the coating with K2TiF6 addition exhibited the best corrosion resistance. Furthermore, the correlation of the electrolyte composition, microstructure and properties had been revealed.

Dependence of the infrared emissivity on SiC content and microstructure of microarc oxidation ceramic coatings formed in Na2SiO3 electrolyte

High emissivity coatings have been widely used in thermal protection systems and high heat efficiency devices in furnaces. In the present work, high emissivity ceramic coatings incorporated with different SiC contents have been prepared on Ti2AlNb alloy by microarc oxidation (MAO) method. The addition of different SiC contents into the Na2SiO3 electrolyte has a distinct influence on the microstructure and normal spectral emissivity of composite coatings. Microstructure of MAO coatings was investigated by using a glancing incidence X-ray diffractometer, a scanning electron microscope, an eddy current thickness meter and a contact surface profiler. A two-step voltage-controlled technology was also used to tailor the microstructure by adjusting the microarc oxidation duration. The composite coatings formed on Ti2AlNb alloy exhibit a porous structure, and consist of R-TiO2, A-TiO2, Al2SiO5, SiC as well as amorphous phase. The normal spectral emissivity of both coated and bared Ti2AlNb samples was measured at 600°C in the infrared wavelength range of 3–20μm. The S-10C composite coating incorporated with a 10g/L of SiC in the Na2SiO3 electrolyte has a much higher emissivity in the wavelength range of 3–20μm than the bared Ti2AlNb alloy. High emissivity of the S-10C coating is attributed to its porous structure and chemical composition, especially SiC content.